[WebMIDI] Introduce MidiManagerUsb

Introduce MidiManagerUsb, a MIDIManager subclass for USB-MIDI.
Android port will use this class.

BUG=303596
R=toyoshim@chromium.org

Review URL: https://codereview.chromium.org/107163008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247665 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247665